Forests are essential to combating climate change. They drink up huge amounts of planet-heating carbon from the atmosphere and provide shelter for species struggling to adapt to global warming. For that reason, experts have called for measures to protect forests. But what about trees in cites? We hear much less about them. Yet the trees that line streets and backyards are just as important as those in the forest — actually, maybe even more so. And we are losing them, too.
New research suggests that American cities and their surrounding areas have been losing as many as 36 million trees a year. That might not sound like a lot when you think about the number of trees in our nation’s forests, but those trees have a powerful impact on health and well-being — and on climate change.
“Even though urban areas are small compared to rural areas, the value of trees is more important because these trees are in more proximity to people,” said David Nowak, a scientist with the U.S. Department of Agriculture’s Forest Service. “This loss may not seem huge, but it’s substantial in proportion to people.”
In addition to soaking up carbon dioxide, trees clean the air of pollutants by absorbing gases and trapping noxious particulates in their leaves and bark. They cool buildings by providing shade and warm them by lowering wind pressure. They also can reduce noise and slow floods.
These trees matter greatly because they are where people are.
“The trees in Montana might remove more air pollution than the trees in New York City, but the trees in New York City are more valuable because they are cleaning the air where people breathe, and reducing energy and air temperatures where people live and work,” Nowak said. “More than 80 percent of the U.S. population lives in urban areas. As a result, those trees are critical in terms of human health and well-being.”
When cities lose trees, they are deprived of the associated benefits. “These benefits include air pollution removal — and its effects on human health — carbon sequestration and energy conservation, that is, the reduced amount of energy being used because of trees,” Nowak said. The loss of trees is costing cities tens of millions of dollars each year.
Nowak and his colleague, Eric Greenfield, both from the Forest Service’s northern research station, used aerial photography to compare 1,000 random geographic sites in each state between 2009 and 2014. They found that tree cover in cities, towns, villages, and suburbs declined by 175,000 acres of tree cover annually, according to the study, which appears in the journal Urban Forestry & Urban Greening.
A total of 45 states showed a net decline in tree cover. Georgia, Florida and Alabama suffered the greatest loss in tree cover, while Mississippi, Montana and New Mexico saw slight increases. Nationally, Maine had the most tree cover in urban areas, while North Dakota had the least, the study said.
“We don’t know what the driver is in every case,” Nowak said. “In some cases, we don’t know why a tree is gone. It could be old age, injury, disease, or choice — someone decides to cut a tree down. But in many cases, it’s development.” Climate change is also contributing to urban tree loss. Severe storms roar through and topple trees, while warming temperatures fuel the spread of tree-destroying insects.
Nowak said that city dwellers can slow the decline of tree cover by planting new trees, but that doesn’t entirely solve the problem. “When you lose a large tree, it makes a huge gap,” he said. “We have a lag effect because it’s hard to replace a large tree with another large tree. Usually, we replace these large trees we’ve lost with new, smaller trees, so it takes more time to fill in that gap. We tend to get pulses of loss — a storm comes through, or out West it could be a fire or insects — with instantaneous losses. We can lose rather quickly, but it’s a slow process to gain, since small trees take 25 to 50 years to reach maturity.”
How should cities cope? “We have to better understand what we have in urban areas — appreciate the value of the trees — and understand how things are changing,” Nowak said. “There needs to be more discussion about what cities want their future to be. Decide what you want to sustain — then make a plan to sustain it.”
Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.
source : https://www.popsci.com/city-trees-are-disappearing#page-5